The tsunami caused by the earthquake washed away hundreds of wooden and bamboo homes, killing over 100 people and displacing 20,000 more. Photograph: Mast Irham/EPA

The combined death toll from the twin tsunami and earthquake disasters which struck Indonesia within 24 hours has risen to more than 300, government officials said today, amid fears it could rise further still as the country's emergency response is stretched to the limit.

Rescuers discovered 282 bodies in the remote Mentawai islands, which were inundated with a 3 metres (10ft) wave when the Pacific "ring of fire" fault line ruptured on Monday night causing a 7.5 magnitude earthquake. World Vision, an aid agency, estimated more than 500 people remain unaccounted for and 7,000 households have been affected.

Around 800 miles away on the island of Java, rescuers scouring Mount Merapi, which erupted on Tuesday, said they had so far discovered 30 bodies, including an 83-year-old man called Maridjan who watched over the volcano's spirits.

He had refused to evacuate the area, saying he would only leave his post if the king ordered it. He was reportedly found kneeling face-down on the floor in a typical prayer position. Seventeen others were taken to hospital.

As the weather cleared over the Mentawai island chain the true scale of the devastation caused by Monday night's tsunami became apparent. The tectonic fault that caused the Boxing Day tsunami of 2004 which killed more than 200,000 people had ruptured again, triggering a giant wave that devastated parts of the island chain and killed hundreds.

The break in the poor conditions that had prevented the remote islands being reached by boat allowed the first cargo plane loaded with tents, medicine, food and clothes to land, but it also bought grim news of the human cost.

Images from the village of Muntei Baru-Baru showed dozens of bodies, including women, young children and babies laid out in the open, some in makeshift bodybags stitched from black plastic sheeting, others in rows sheltered only by corrugated zinc sheeting torn from roofs.

The damage to the area's basic infrastructure also became apparent. Buildings were wrecked, their walls and roofs buckled and broken, and trees were flattened by a wave which has been described as between 3 and 7 metres high and a kilometre wide. Hundreds of lightweight homes were washed away and aid workers reported villages razed to the ground as waves reached 600 metres inland.

"Finally we have a break in the weather," said Harmensyah, head of the West Sumatra provincial disaster management centre, who like many Indonesians only uses one name. "We have a chance now to look for the missing from the sky and also to survey the damage."

The National Disaster Management Agency said most of the deaths occurred in Pagai Utara and Pagai Selatan districts. Officials said hundreds of wooden and bamboo homes were washed away in more than 20 villages, displacing more than 20,000 people. Many were seeking shelter in makeshift emergency camps or with family and friends.

Eight Australians, an American and a New Zealander arrived in the Sumatran city of Padang and told of the moment the wave struck their anchored boat.

They said they were on the back deck of the MV Midas when a wall of water smashed them into a neighbouring boat, triggering a fire that quickly ripped through their cabin. "They hit us directly in the side of the boat, piercing a fuel tank," said Daniel North, the American crew member. Other surfers told of clinging to their boards when the wave swept through them.

At the inland village of Sikakap, hundreds of people waited for medical aid. Alan Rogerson, logistics manager of Surf Aid, a charity which operates in the Mentawai islands, said: "A couple of people died while our staff were talking to them. People have been dragged over rough ground and been hit by debris."

In Peurogat, a village of 104 families, 38 people were killed and 15 were still missing. In Beleerakso 18 were dead and four were missing, said Eloi Bonas, a consultant with the Catholic run Caritas Fund.

"It's a lot more serious than we thought at the beginning," said Dr Dave Jenkins, founder of Surf Aid. "Of the coastal villages that faced the epicentre, probably 50 to 60% of the villages have been affected and lots of them have just been wiped out. There's a lot of a destruction and there's a lot of people who aren't being treated."

In Java there were fears for the safety of people who live beside Mount Merapi or Fire Mountain, which erupted on Tuesday and threatens to do so again. Many residents who had evacuated following the eruption which claimed at least 30 lives, ignored official warnings and tried to return to their homes and crops.

"A lot of energy is pent up back there. There's no telling what's next," said Surono, the head of Indonesia's centre of volcanology and geological hazard mitigation.

Indonesian Red Cross said more than 36,000 people have been evacuated from the mountain where in several areas everything was caked with ash.

The blast eased pressure that had been building up behind a lava dome perched on the crater. But experts warned that today's relative calm could be a lull in the volcanic activity, the dome could still collapse, causing an avalanche of the blistering gas and debris trapped beneath it.

President Susilo Bambang Yudhoyono cut short a state visit to Vietnam to deal with the disasters. "I want to make sure the emergency response has been running well," he told reporters in Hanoi. "I want to see the condition of the victims."

Tragic coincidence

The earthquake and volcano that struck Indonesia lie on the same volatile fault line known as the Pacific ring of fire, but it was simply a tragic coincidence they occurred within a few hours of each other, said experts.

Dr Colin MacPherson, a reader at Durham University and an expert on volcanic activity in Java, said it was not feasible that the earthquake prompted the eruption.

He pointed out that Mount Merapi, 800 miles from the earthquake, is one of the world's most volatile volcanoes, has erupted several times over the last century, but never just after an earthquake. "There is no one-to-one correspondence between earthquakes and volcanic eruptions," he said.

For a volcano to erupt, molten rock comes to the surface in a process that takes hundreds of years, MacPherson explained. He said the volcanoes on the islands were created by the same seismic activity that makes Indonesia so vulnerable to earthquakes.